This is likely a co-planarity problem with the parts. Some parts have lead displacements before you place them. Many fine pitch placers have a co-planartiy option.

The machine will inspect the leads on each part prior to placement. Several of our Juki systems have this option.

Are the leads lifted or has the solder wicked up the lead leaving the pad/lead interface dry?

If this is the case it is likely the differential heating of the leads - the lead elbow/shoulder heating faster than the foot. This can be traced back to reflow profiling and solder deposition issues.

There are a few things to review, the flatness of the pads in relationship to each other, the co-planarity of leads of each component and the amount of solder paste deposited on the land areas.

Boards which have been hot air leveled (HASL) will have a rough topography on each pad and if the component is place on one of the high spots it may allow the lead enough height to miss being soldered and exhibit itself as a lifted lead problem.

Using immersion plating such as an OSP coating, or an ENIG plating will make the pads much flatter in relationship to each other and will help in reducing and eliminating this problems.

As for the components, an audit may be in order to determine if the components are within the acceptable tolerances of the component co-planarity requirements.

As for the paste deposition process, check the amount or height of paste deposited and the size of the aperture on the stencil.

I have no data on fall out rates so I can't comment however I have seen what sounds like the same problem before, in these cases it was due to a coplanarity issue with the problem leads.

To be sure I suggest a cross section on the affected lead and adjacent leads to look at the stand off of the lead from the pad.

What may help is a solder paste with an extended activity life so that when the component is pulled down on to the solder when it wets the other leads there is still sufficient activity to clean the problem lead and allow the molten solder to wet the lead.

At all times during the handling and processing of QFPs, special care must be taken to make sure that all the leads are co-planar. QFP leads are small and very fragile and the smaller the lead pitch the more fragile the lead. Any mishandling during prior processing renders the leads prone to lack of co-planarity.

It is most unlikely that the act of reflow soldering wil cause a lead to "lift". There is a much greater probability that a lead has become bent upduring one of the many handling functions to which the component is subjected , i.e.the cropping, loading, unloading, transport, picking and placing etc.,of the components.

Suspect components should be inspected prior to reflow to check for co-planarity or lack thereof. However, with less than 1% fall out it may be an arduous task to identify defective components.

The lifted leads may very well be a lack of perfect planarity of the leads, in that all the leads are not in the same horizontal plane. Each component actually has a co-planarity specification, which is typically around 100 microns, but could be larger depending on the type of QFP package.

 It is also possible to experience damage to the pins, especially the corner pins, since they are the most likely to be damaged during handling. The ratio of the solder paste height and the lead coplanarity could be problematic, in that if the solder paste height is modest, there may not be enough solder volume to "catch" the worst non compliant parts.

A slight overprint of the solder paste could provide the small additional solder volume. As the paste reflows, it pulls onto the exposed pad, and since the pad controls the x-y coordinates, the solder will rise in the z direction, and catch the non-compliant pin.

Another option is to place a small solder preform with a chip shooter out of tape and reel packaging into the solder paste if room for overprinting on the card is not available.